Electric circuit



1945- z. 0. ST. PALLEY 2,366,579

ELECTRIC CIRCUIT Filed May 23, 1944 3 Sheets-Sheet l L u R T) ggzgqongg oonoogmo r-N L; a

Inventor: Zoltan O. StPal ley,

y His Attorney.

Jan. 2, 1945. I z, 51 PALLEY 2,366,679

ELECTRIC CIRCUIT Filed May '25, 1944 I 3 Sheets-Sheet 2 Inventor: Zoltan O. StPal ley,

H is Attorney.

Jan. 2, 1945. 2. 0. sn PALLEY ELECTRIC CIRCUIT Filed May 23', 1944 s Sheets-Sheet 5 mifooocloooo Fig.5.

R t Rich-.13: TCurr-ant I Adjiszr 0z2donTap Closed Closed #2. [I1 and 55 57 #2 IY and 55 58 2. y incl 55 59 SZI #5 55 59 'SZII #3 54 9 Inventor:

H is Attorney Patented Jan. 2, I945 ELECTRIC CIRCUIT Zoltan 0. St. Palley, Pittsfield, Mass., asslgnor to General Electric Company, a corporation oi New Y rk Application May 23, 1944, Serial No. 536,873

15 Claims.

This invention relates to electric circuits and more particularly to improvements in tap-changing circuits of theso-called load-ratio-control variety. This is a continuation-in-part of my application Serial No. 443,561, filed May 19, 1942, entitled Electric circuit."

One way to change the eflective number of turns of inductive apparatus, such as a reactor or transformer winding, is, to provide it with a plurality of taps and tap 'changing switches for selectively making connection with the taps. In transformers in which this is done without interrupting the load current it is called "load-ratiocontrol." Such circuits usually have at least two tap-changing switches, or their equivalent, which are so operated that at least one of them always makes connection to a tap. In such circuits, before any one of the two switches is operated to make a tap change, the other one is caused to make or has made connection with the same, or an electrically adjacent, tap. The latter connection is often referred to as the bridging connection or half-cycle position of the circuit. in this position the load current is divided substantially equally between the two switches by means oi autotransformer because it transforms the circuit voltage to a voltage intermediate the voltage of the bridged taps. It will thus be seen that whenever a bridging connection is broken by the opening of a tap-changing switch at least half the main load current must be interrupted, such interruption resulting in the current being transthese contacts. Thus an arc is formed and the arcing duty of the switch is determined by the size of the contact current before opening and the voltage across them after opening.

justlng switches are usually mounted inside the transformer tanks so as to make it unnecessary to bring the relatively large number of tap connections through the tank. However. such transformer tanks are often filled with insulating and cooling liquid whose insulating property is seriously impaired by arcing therein. It is therefore imperative that arcing ofratio adjusting switches which are located in such liquid be prevented.

One way t do this is to bring out a lead from each ratio adjuster switch and connect it in series with an individual arcing duty contactor located outside the transformer tank. By opening each contactor before its associated ratio adjuster is opened or closed the latter will only switch a dead circuit and no arcing can take place in the tank. However, arcing, even out ide the tank, is not desirable and arcing contactors have to be expensively constructed in order to be able to interrupt short circuit currents.

In accordance with this invention there is provided a new and improved load-ratio-control cirsuit in which the arcing duty on the ratio ad-. iusting switches is reduced to zero with respect to both voltage and current. In one form of the invention this is accomplished without the use of any additional switches so that no arcing takes place anywhere in the circuit. In other forms of the invention auxiliary means including relatively low arcing duty contactors are used for reducing to zero the current, or the voltage, or. both, of the ratio adjuster contacts before and after they make a tap change. The invention is characterized by separate auxiliary means for reducing the current in the ratio adjusters to zero before they make a tap change and for reducing the voltage across the tap-changer contacts to zero after they break their connection with a tap.

An object of the invention is to provide a new and improved electric circuit.

A further object of the invention is to provide a new and improved tap-changing circuit.

A further object of the invention is to provide a new and. improved load-ratio-control circuit.

An additional object of the invention is to provide an improved arrangement for reducing substantially to zero the arcing duty on the ratio adjusting switches of a load-ratio-control circuit.

The invention will be better understood from the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings, Fig. 1 illustrates diagrammatically a preferred embodiment of the invention, Figs. 2, 3, 4, 5, 6 and 7 show successively the changes in position of the parts of Fig. 1 during its operation, Fig. 8 is a modification showing a different connection of the means for reducing the voltage component of the arcing duty of the ratio adjusters and showing a tap-changing arrangement for controlling the current component of the arcing duty of the ratio adjusters, Figs. 9

- more windings.

and show different positions of the means in Fig. 8 which is used for controlling the voltage component of the arcing duty of the ratio adjust ers, Fig. 11 is another modification in which a vernier-type load-ratio-control circuit is provided with a commutator-type tap changer for controlling the current component of the arcing duty of the ratio adjusters, Fig, 12 is a further modification in which separate arcing duty tap-changing means are employed for controlling both the voltage and current components of the arcing duty of the main ratio adjusting switches, and Fig. 13 is a chart which defines the various operating positions of Fig. 12.

Referring now to the drawings and more particularly to the first seven figures thereof, there is shown therein a main winding I provided with a plurality of equal electrically spaced taps 2,3, 4,

5, 6 and l. Winding I may be a part of any inductive or reactive apparatus and is typically one of the windings of a transformer, particularly a relatively large-size relatively high-voltage power transformer. Associated with the taps is a pair of ratio adjusting switches 8 and 9. In the arrangement shown, the ratio adjuster 8 makes and breaks connection selectively with the even'numbered taps and the ratio adjuster 9 similarly cooperates with the odd-numbered taps. This, however, is not essential and both ratio adjusters can be made to make and break connection with the same tap, if desired. The main or load circuit of ,the winding E is divided into two effectively parallel branches l0 and Il which terminate at one end with the ratio adjusters 8 and 9 respectively and which terminate at the other end in a common connection to one side or conductor l2 of the main circuit of the winding. The other side or conductor of this circuit is designated by ES.

The connection of the branches I0 and H to the common conductor I3 is completed through a pair of adjustable devices l4 and I5, both of which have the form of an induction voltage regulator. Such regulators are characterized by a stator and a rotor, both of which carry one or In the case of the device it there is a pair of equal windings l8 and H which are fixed relatively to each other and there is a third winding l8 which is relatively rotatable with respect to the windings l8 and H. The winding l8 may be mounted on either the rotor or the stator, in which case the other two windings will, of course, be mounted on the other core member. This regulator is used to reduce to zero the voltage component of the arcing duty of both of the ratio adjusters 8 and 9.

The regulator i5 which serves selectively to reduce to zero the current component of the arcing duty of the ratio adjusters 8 and 8 is similar to the regulator M in that it has a pair 01' relatively fixed windings l9 and and a winding 2| which is relatively rotatable with respect to the windings l9 and 20, and like the regulator l4, winding 2! may be on either the stator or the rotor. The windings I8, 28 and 2| preferably all have the same number of turns. For the purposes of description it will be assumed that both the windings l8 and 20 are on the rotors of their respective regulators; I

The winding i8 is excited by any suitable means; thus, for example, it can be connected across a suflicient amount of the main winding l to give the proper exciting voltage. This voltage is one in which when the winding I8 is rotated so as to induce the maximum voltage in the windings l8 and II, this maximum voltage in each case will be equal to one-halt the voltage between adjacent taps'of the main winding;

The connections of the remaining regulator windings are such that the windings l6 and I8 are in series with each other in the branch H and the windings l1 and 20 are in series with each other in the branch ill, the winding 2| being in series with the main conductor l2.

The regulators I4 and I5 may be mounted either inside or outside of the tanker casing for the main transformer winding. In the latter case, the branch conductors l0 and II will have to be brought out of the tank through insulating bushings but in the former case only the conductor I! need be brought out through an insulating bushing regardless of whether or not the conductor I3 is brought out through a bushing or is grounded to the tank. Also, the ratio adjusters and the rotors of the regulators l4 and I5 may be separately driven or may be driven by any suitable common operating means. The arrows associated with the windings are for the purpose of indicating the relative polarities of their voltages.

The operation of Fig. 1 is as follows: Ratio adjuster 8 is making connection with tap 2 and it is assumed that it is desired to have ratio adjuster 8 make connection with tap 3. The winding 48 is shown in its position of minimum or zero coupling with respect to the windings l6 and I! so no voltages are induced in the latter and consequently the voltage of the conductor I2 is substantially the same as the voltage of the tap 2. The winding 2| of the regulator I5 is in its position of maximum coupling with respect to the winding 20 and is in the position of minimum coupling with respect to the winding l9. Consequently, the impedance of the regulator l 5 to any load current in the circuit l2-l 3 is very low, being merely the leakage reactance of the closely coupled windings 20 and 2 I.

Referring now to Fig. 2, the winding [8 has been shown rotated ninety electrical degrees so as to induce maximum voltages in the windings l6 and I! in such directions that their polarities are opposite to the polarity of the voltage between the taps 2 and 3. As each of these voltages is equal to one-half the tap-to-tap voltage difference, it follows that their sum cancels the tap-totap voltage difference in the serie circuit including both tap changers, the windings i6 and I1 and the portion of the main winding between the taps 2 and 3. In other words, the voltage between the tap 3 and the ratio adjuster 8 has been reduced to zero. Consequently, the tap changer v 8 can be moved into contact with tap (I, as shown in Fig. 3, without any arcing as there is neither current flow between the contacts nor a voltage v diiTerence between them.

equal to the voltage induced in the winding l1.

Thus, by rotation of the winding l8 through intermediate angles, the voltage of the conductor I2 may be smoothly raised and lowered, depending upon the direction of rotation of the winding it, a maximum amount equal to one-half the tap-to-tap voltage difference. In this manner smooth vernier-like control of the voltage of the circuit |2--l3 may be secured. As the voltage of the conductor I! in Fig. 2 has been lowered to a voltage midway between that of tape 2 and I, the movement of the ratio adjuster into engagement with the tap 8 in Fig. 3 will not change the voltage of the conductor I2. This, of course, also follows from the fact that the movement of the ratio adjuster 9 into contact with the tap 3 is done at zero voltage and current so that no changes in the electrical conditions of the circuit take place.

The position of the tap changers in Figs. 1 and 2 is often referred to as the quarter-cycle position because only one ratio adjuster is carrying current, while the position of the ratio adjusters shown in Fig. 3 is often'referred to as a halfcycle or bridging position because the voltage of the common conductor I2 is usually half-way between the bridged taps. I

If now the winding 2| is rotated away from the position shown in Fig. 3, its coupling with respect to the winding 20 will decrease until when it reaches the opposite extreme position shown in Fig. .4 it becomes closely coupled with the winding I9 and is practically entirely uncoupled with respect to the winding 20. During this movement of the winding 2| the load current in the main winding circuit is gradually shifted from ratio adjuster 8 to ratio adjuster 9 so that with the winding 2I in the position shown in Fig. 4 substantially all of the load current flows through the ratio adjuster 9 and none of it flows through the ratio adjuster 8. This is because the magnetizing reactances of the windings I9 and 20 cancel each other so that only the relatively low leakage reactance is opposed to the flow of current through the ratio adjuster 9, whereas the relatively high magnetizing reactance of the winding 20 opposes the flow of current through the ratio adjuster 9. Consequently, the voltage of conductor vI2 is now substantially the same as the voltage of tap 3.

With the winding 2| in positions intermediate those shown in Figs. 3 and 4 the current through the tap changers 8 and 9 will divide in any desired ratio and likewise the voltage of the conductor I 2 will vary anywhere between the voltage of tap 2 and the voltage of tap 3. The regulator 4 thus serves as a smooth Vernier voltage adjuster for the main circuit when the ratio adjusters are in their bridging or; half-cycle position. In this half-cycle position the voltages of the windings I6 and I! have no effect on the voltage of the conductor I2 because they are inserted in the bridging circuit symmetrically with respect to the conductor; that is to say, they are inserted in equal amounts in both the branch circuits I -andII.

Ratio adjuster 8 can now safely be moved out of engagement with tap 2 without the occurrence of any arcing because no loadcurrent is flowing through the ratio adjuster B and because no voltage will appear between the adjuster 8 and the tap 2 when they separate since the sum of the voltages of the windings I6 and I1 is equal and opposite to the voltage between the taps 2 and 3. It should also be noted that in the positions shown in Figs. 3 and 4 the regulator windings I6 and I1 draw no magnetizing current which has to be broken by the ratio adjuster 8 because the sum of the voltages of the windings I6 and I1 being equal and opposite to the voltage between the taps 2 and 3, there is no net voltage for exciting them.

The winding I8 is now reversed, as shown in Fig. 6, so as to reverse the voltages induced in the windings I6 and IT. This is in preparation for the ratio adjuster 8 making contact with the tap 4 whose voltage has the opposite polarity with respect to tap 3 that tap 2 has. Consequently,

with the winding I8 of the regulator II in the position shown in Fig. 6 the voltage between the ratio adjuster 8 and the tap 4 is zero so that these two parts may be brought into contact without any arcing taking place.

From the above description it will be obvious that by alternately operating the regulators I4 and I5 the ratio adjusters may be operated along the whole row of taps without at any time having to perform any arcing duty.

In Fig. 8 the ratio adjuster 8 is replaced by a series of contacts 22, 23 and 24 for selectively connecting the even-numbered taps to the branch circuit I0 and the ratio adjuster 9 is replaced by contactors 25, 26 and 21 which serve selectively to connect the odd-numbered taps to the branch circuit II. The regulator I4 is a simple two winding induction regulator having a. single secondary winding I6 mounted on the rotor and connected between the branch circuits In and II. The primary winding I8 is mounted on the stator and is connected across a portion of the main winding i.

Regulator I5 oi Figs. 1-7 is replaced by an auxiliary tap-changing transformer 28 having a pair of windings 29 and 30 corresponding respectively to the windings i9 and 20 of Figs, 1-7 and having a primary or exciting winding 3! permanently closely coupled to the windings 29 and 30 as is the case with most stationary winding transformers. This auxiliary transformer is excited by means of a series transformer 32 having a winding 33 connected in series with the conductor I2 and having three secondary windings 34, 35 and 36, all three of which are interconnected at a common point 37. The remaining terminal of the winding 34 is permanently connected to an intermediate point on the winding 3|, such, for example, as its mid-point, and the remaining terminals of the windings 35 and 36 are connected respectively to sets of contactors for selectively making connection to oddand even-numbered sets of taps brought out of the winding 3|. The three windings 33, 35 and 36 together constitute what may conveniently be referred to' as a forked winding, the winding 34 corresponding to the main section or handle of the fork andthe windings 35 and 36 being auxiliary sections which correspond to the tines of the fork. The terminals of the windings 35 and 36 which are connected to the point 31. have the same instantaneous polarity and the terminal of the winding 34 which is' connected to the point 31 preferably has the opposite instantaneous polarity so that ordinarily the polarities of the windings 34 and 35 in series are additive and the polarities of the windings 34 and 3B in series are additive and the polarities of the windings 35 and 36 in series are subtractive. The voltage of winding 34 is ordinarily, although not necessarily, substantially greater than the voltages of the windings 35 and 36 and the voltages of the windings 35 and 36 are ordinarily, although not necessarily, equal to each other. When they are unequal the difference between their voltages produces an effect corresponding to that of a tickler winding and thus increases the number of steps which can be obtained with a given number of taps in the winding 3|.

The operation of Fig. 8 is as follows: The ratios of the transformers 28 and 32 are so pro-portioned that with the contactors in the positions shown the current'circulated by the transformer 32 in the portion of the winding 3| most closely coupled to the winding 30 will be equal and opposite to the ampere turns of the winding 30 when all of the load current in the circuit |2--|3 flows through the branch circuit Ill. Therefore, by transformer action the current in the winding 30 is forced to be equal to the total load current so that all of the current flows through the ratio adjuster contactor 23. Winding |5 is then rotated into inductive relation with winding IS, as shown in Figs. 9 and 10, so as to induce a voltage in the winding l6 which is equal to the voltage difference between taps of the main winding I and this voltage is impressed in series with the tap voltage difference and with the closed contactor 23. The polarity of the voltage of winding I6 is opposite in Figs. 9 and 10 so that either contactor or contactor 26 may be closed without there being any voltage difference between their contacts providing that the winding |6 i in the proper position so that the polarity of its voltage is equal and opposite to the voltage difference between tap 4 and tap 3 or tap 5, a the case may be. After contactor 25 or 26 is closed the load cur rent is transferred to branch circuit II by progressively closing the auxiliary contactors cooperating with the taps of the winding 3| in a right to left direction and progressively opening the ones that have been closed after the next lefthand one has been closed so as to prevent opening of the secondary circuits of the series or current transformer 32 as the opening of such circuits would produce dangerous voltages. two adjacent auxiliary contactors are closed the windings 35 and 36 act as a preventive reactor and limit short circuit currents. Unlike a pre-v ventive reactor, however, it is the leakage reactance of the windings 35 and 3B which serves to limit the short circuit or circulating current. Thus, in effect, it is an air core rather than an iron core reactance which limits the circulating short circuit current when adjacent auxiliary contactors are closed. When the left-hand auxiliary contacts close and all the others open, the load current will all be transferred to the branch circuit l and ratio adjusting contactor 23 may then service. This transformer is provided with a pri mary winding 38 connected by means of a pantograph 39 between an overhead trolley wire or When third rail (not shown) and ground- This equal to one-half the tap voltage difference of the vemier winding 40 and is for the purpose of giving twice as many vemier voltage steps as there are vernier voltage taps.

The primary winding |8 of the regulator I4 is connected across the vernier winding and the secondary winding I! is connected to impress its voltage between the branch circuits l0 and II, as in Fig. 8, so that the tap voltage difference 7 of th main winding may be cancelled before the operation of each of the main winding ratio adluster contacts.

The current transfer means is a commutator arrangement 44 comprising a commutator member 45 having one set of segments connected to the right-hand half of winding 3| and having another set of segments connected to the lefthand half of the winding 3|. Contacting this commutator are movable brushes 46 and 41 connected respectively to the terminals of a conventional current transformer 43 connected in the conductor |2. With the brushes in the position shown, the current transformer secondary winding is short circuited. Movement of the brush from the position shown in either direction willprogressively increase the number of turns of the rightand left-hand halves of the winding 3|, as the case may be, through which the secondary winding current of the current transformer 43 flows, thereby causing corresponding ampere turns of energization of the windings 29 and 33, as the case may be, so as to cause current division between the branches I0 and II to vary between a maximum for one and zero for the other and vice versa. With the parts in the position shown the currents will be equal. Operation of the brushes 46 and 4'! in combination with regulator |4 results in operation of the main contactors without any arcing, a will be obvious from the description of the preceding figures.

In Fig. 12 the voltage reducing induction regulator 4 is replaced by an auxiliary tap-changing circuit comprising a transformer having three relatively fixed windings 49,50 and 5|, the windings 49 and 50 being connected in the branch circuits l0 and II respectively and the winding 5| being connected by means of a mid-tapped reactor 52 and auxiliary contactors 53, 54 and 55 to various portions of a tertiary winding 58 of the main transformer. With the auxiliary contactor 54 closed, the winding 5| is effectively short circuited so that no voltages are induced in the windings 49 and 52, whereas closure of contactors 53 or 55 will connect the winding 5| across different halves of the tertiary winding 56 with reverse polarity and the ratio of turns of the windings is such that the voltages induced in the windings 49 and 50 are equal respectively to one-half the tap voltage difference of the main windin'g.

The current transfer circuit is similar in principle to the corresponding circuits of Figs. 8 and 11 except that only three contactors 51, 53 and 53 are employed and they are connected by the secondary winding of current transformer 48 to a mid-tapped reactor III for preventing open-circuiting of the current transformer 48 when a tap change is made on the winding 3| by the contactors 51, 53 and 59. a

The operation of Fig. 12 is the same in principle as the operation of the preceding circuits, the transformer of the current from one main winding ratio adjuster being secured by the selective operation of the contactors 51, 53 and 59 and the reduction of th voltage difference between the ratio adjusters 3 and 3 and the taps with which their connections are made and broken bein controlled by the auxiliary contactors 54 and 55. The table shown in Fig. 13 sets forth a complete cycle of operation of Fig. 12 and the detailed operation of this circuit will be clear from a consideration of this table in the light of the description of the preceding figures.

While there have been shown and described particular embodiments of this invention, it will be obvious to those skilledin the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention,

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a winding having a plurality of taps between electrically adjacent pairs of which there is substantially th same tap voltage difference, a load circuit for said winding, said circuit being selectively completed through either one or both of a pair of ratio adjuster contacts which selectively make connection separately and simultaneously to respective ones of an adjacent pair of said taps, separate voltage regulator means for inserting in series with either ratio adjuster contact a voltage substantially equal and opposite to said tap voltage difference before its connection with a tap is made or broken, and means for transferring substantially all the current in said load circuit to either ratio adjuster contact before the connection of the other ratio adjuster contact with a tap is made or broken whereby said ratio adjuster contacts make and break their connections at substantially zero voltage and current.

2.. In combination, a transformer winding having a plurality of electrically equally spaced taps, a load circuit for said winding having a pair of effectively parallel branches, a pair of ratio adjusting contacts for selectively connecting corresponding ends of said branches to said taps, a pair of auxiliary windings connected respectively between the otherv ends of said branches and said circuit, means for varying the effective relative reactance of said auxiliary windings so as to shift substantially all of the load current in said circuit from one ratio adjuster contact to the other, and separate voltage regulator means for impressing in series with said ratio adjusting contacts a voltage equal and opposite to the voltage difference between said taps.

3. In combination, a main transformer winding provided with a plurality of taps, a pair of nonarcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrically adjacent taps, means for transferring all the load current of said main transformer winding to each ratio adjuster before the other one makes and breaks said bridging connection, and separate voltage regulator means for impressing across the contacts of each ratio adjuster a voltage equal and opposite to the voltage difference between bridged taps of said main winding before said bridging connection is made and broken. 4. Incombination, a main transformer winding provided with a plurality of taps, a pair of nonarcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes makin and breaking a bridging connection between electrically adjacent taps, first and second induction voltage regulators each having a pair of relatively fixed windings and a third Winding rotatable with respect thereto, the relatively fixed windings of said first regulator being connected respectively in series with said ratio adjusters, the third winding of said first regulator being connected in series with both of its relatively fixed windings so as to be traversed by the entire load current of said main transformer winding, the relatively fixed windings of said second regulator being connected respectivel in series with the relatively fixed windings of said first regulator,

and connections for exciting the third winding of said second regulator.

5. In combination, a main transformer winding provided with a plurality of taps, a pair of non-arcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrically adjacent taps, an induction regulator having apair of relatively fixed windings connected respectively in series with said ratio adjusters, and a third winding on said regulator rotatable with respect to its other two windings, said third winding being connected in series with each of its relatively fixed windings with respect to the entire main transformer load current.

6. In combination, a main transformer winding provided with a plurality of taps, a pair of non-arcing duty ratio adjusters for selectively making connections to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrically adjacent taps, a winding provided with a mid-tap for completing said bridging connection and completing the load circuit connections of said ratio adjusters, a variable voltage transformer having a primary and a secondary winding, said secondary winding being connected between said ratio adjusters and across said winding which is provided with a mid-tap, and connections for exciting said primary winding' 7. In combination, a main transformer winding provided with a plurality of taps, a pair of non-arcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrically adjacent taps, a winding provided with a mid-tap for completing said bridging connection and completing the load circuit connections of said ratio adjusters, a multi-tapped auxiliary winding in inductive relation with said midtapped winding, a current transformer having a primary winding connected in series with said mid-tap, said current transformer having three secondary windings, a terminal of one of said secondary windings being connected to an intermediate tap in said auxiliary winding, the other terminal of said last-mentioned secondary winding being connected in a terminal of each of the remaining secondary windings, and means for selectively connecting the other terminals of said remaining secondary windings to alterate sets of taps in said auxiliary winding.

8. In combination, a main transformer winding provided with a plurality of taps, a pair of nonarcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrically adjacent taps, tap-changing means for shifting the load current of said main transformer to oneratio adjuster before the other one makes or breaks said bridging connection, and an induction regulator connected to reduce the voltage across the contacts of said other ratio adjuster substantialfy to zero after it breaks said bridging connection and before it makes said bridging connection.

9. In combination, a winding having a plurality of taps, tap changers for selectively making and breaking connection with said taps, three windings connected together at a common point, a fourth winding inductively coupled to each of said three interconnected windings, means for connecting one of said three windings to an intermediate point in said tapped winding, and means for connecting the other two of said three windings to said tap changers respectively.

10. In combination, a main transformer winding provided with a plurality of taps, a pair of non-arcing duty ratio adjusters for selectively making connection to certain of said taps in a predetermined sequence which includes making and breaking a bridging connection between electrlcally adjacent taps, supplementary transformer tap-changing means for transferring all of the load current of said main winding to one of said ratio adjusters before the other one makes and breaks said bridging connection, and additional transformer tap-changing means for reducing to zero the voltage between the contacts of said ratio adjusters while said contacts are separated.

11. In combination, a load-ratio-control transformer winding having a plurality of taps, a pair of non-arcing duty ratio adjuster switches for selectively making connection to said taps, a circuit for said winding normally having said switches effectively connected in parallel circuit relation therein, said switches alternately having a half-cycle position in which they make a bridging connection with respect to a pair of ad- Jacent taps and a quarter-cycle position in which one makes connection to a tap while the other one is between taps, and two separate means for selectively varying the voltage of said circuit when said tap changers are in said half-cycle and full-cycle positions respectively.

12. In a load-ratio-control system, a transformer having a multi-tapped winding, a second transformer having a forked winding which comprises a main section and two auxiliary sections all joined together at a common junction point, tap-changing means for selectively making connection with said taps in a predetermined order, means for connecting said auxiliary sections of said forked winding to said tap-changing means, and means interconnecting said tapped winding and the main section of said forked winding.

13. In a load-ratio-control system, a transformer having a multi-tapped winding, a second transformer having a forked winding, said forked winding comprising three sections two of which are equal, one pair of corresponding instantaneous polarity terminals of said equal sections being connected to the opposite instantaneous polarlty terminal of the remaining section, means for connecting the other terminal of said remaining section to a point on said multi-tapped winding, and tap-changing means for selectively connecting the other terminals of said equal sections to said tape, certain conditions of said tapchanging means simultaneously connecting said other terminals of said equal sections to different taps, said equal sections having a predetermined leakage reactance for limiting the circulating cur rent therein due to the tap-to-tap voltage difference to a safe value under said certain conditions of said tap-changing means.

14. In a load-ratio-control system, an ironcore transformer having a multi-tapped winding, a pair of serially-connected additional windings on another iron core, tap-changing means for selectively connecting the free terminals of said additional windings to certain of said taps in a predetermined order which includes the simultaneous connection of said terminals to different taps, the polarity of said additional windings being such that circulating current in them due to the tap voltage difference when their terminals are simultaneously connected to different taps produces opposed mmfs. in their core whereby said circulating current is primarily limited by the leakage reactance of said additional windings, and an external circuit connected between a point on said multi-tapped winding and the Junction of said additional windings.

15. In a load-ratio-control system, an iron-core transformer having a multi-tapped winding, a pair of serially-connected equal additional windings on another iron core, tap-changing means for selectively connecting the free terminals of said additional windings to certain of said taps in a predetermined order which includes the simultaneous connection of said terminals to eleco trically-adjacent taps respectively, the polarity of said additional windings being such that circulating current in them due to the tap voltage difference when their terminals are simultaneously connected to electrically-adjacent taps produces opposed mmfs. in their core whereby said circulating current is primarily limited by the leakage reactanoe of said additional windings,

and an eirternal circuit connected between a point on saidmulti-tapped winding and the Junction of said additional windings.

ZOLTAN 0. ST. PALLEY. 

